The objective of the proposed investigation is to determine the major hormones and combination of hormones that are primarily responsible for the regulation of lipoprotein lipase (LPL) in heart tissue. This enzyme is the rate limiting step in the hydrolysis of circulating lipoprotein triglyceride and therefore plays an important role in determining the final plasma triglyceride concentrations. Additionally, the activity of the enzyme in various tissues determines the site of uptake of plasma triglycerides. The activity of LPL in the heart is known to be influenced by various nutritional states but the hormones presumed to be mediating these changes are not known. This study will investigate the hypothesis that the levels and proportions of plasma glucagon, corticosteroids, and insulin are primarily responsible for regulating heart LPL activity. These hormones will be measured during nutritional conditions which are known to acutely alter the functional fraction of heart LPL activity. The data obtained from these studies will be used to attempt to modulate LPL activity in vitro in the isolated perfused heart. Development of an in vitro model for the alteration of LPL activity in the heart would allow for study of specific mechanisms of enzyme alteration. Data contributing to the understanding of the regulation of LPL may be of potential significance in the treatment of hyperlipidemias and regulation of other plasma lipoprotein including density lipoproteins (LDL) and possibly the high density lipoprotein (HDL) fraction.